This invention relates to a television signal conversion system for removing redundant fields from a video signal that is produced from a movie film by a 2-3 pull down system or the like, an image encoding system for encoding the video signal, and an image decoding system for decoding the encoded data.
A method of compressing and encoding long-time video signals and recording them on storage media has been examined with the development of digital storage media. The MPEG (moving picture image coding expert group) of the international electric standard conference in the international standard organization (ISO) has been in operation toward the standardization of the moving picture compression and encoding system.
The video signal can be considered to include the following two types. One is a television signal of 30 frames of image per second (NTSC system), and the other is a film of 24 frames of image per second. The 2-3 pull down system is known as a system for converting the frame number of a film into that of a television signal (NTSC).
FIG. 10 shows the principle of the widely used 2-3 pull down system.
The first frame of a film is converted into the first and second fields. The second frame of the film is converted into the third, fourth and fifth fields. The third frame is converted into the sixth and seventh fields. The fourth frame is converted into the eighth, ninth and tenth fields. Thus, such conversions are repeated, resulting in the conversion of the film into a television signal (NTSC system). The video signal resulting from conversing a film into a television signal is called "telecine signal".
The telecine signal obtained by the 2-3 pull down system periodically includes duplicate fields, or redundant fields as will be seen from FIG. 10. In FIG. 10, the third and fifth fields are just the same field (the latter one is the duplicate field), and the eighth and tenth fields are also just the same field (the latter one is the duplicate field).
Therefore, if the telecine signal produced by 2-3 pull down system is directly compressed and encoded, the redundant, duplicate fields are also compressed and encoded. Thus, when the video signal to be compressed and encoded is a telecine signal, the duplicate fields are eliminated from the telecine signal before the compression and encoding process, and then the encoding or transmitting means is used.
FIG. 11 shows the conventional system for detecting and eliminating the duplicate fields of the telecine signal produced by converting a film into a television signal as described above.
Referring to FIG. 11, there are shown an input terminal 10 of the system, a control signal output terminal 20 for outputting a control signal according to the telecine image, and an output terminal 30 of the system. In addition, there are shown a frame memory 100, a subtracter 200, an absolute value circuit 300, an accumulator 400 for producing an accumulated result per field, a comparator 500 for producing results of comparison to a threshold, a hold circuit 600 for holding data for one-field period, and a control signal generator 700 for generating the control signal according to the telecine image. Moreover, there are shown a write control circuit 800 for controlling a memory 900 to write, a read control circuit 850 for controlling the memory 900 to read, and the frame memory 900 for storing the video signal. The operation of the conventional television conversion system constructed as above will be described with reference to FIG. 11.
The telecine signal fed through the input terminal 10 of the system is directly supplied to the subtracter 200 and also to the frame memory 100, by which it is delayed one frame. The delayed signal is supplied to the subtracter 200, where an interframe difference signal is produced. The interframe difference signal is supplied to the absolute value circuit 300 where its absolute value is produced, and then the absolute value is accumulated for each field unit by the accumulator 400. The accumulated result is supplied to the comparator 500, where it is compared with a threshold. If the accumulated result is larger than the threshold, it is decided that the current field is of a different frame of the original film from the frame of the one-frame preceding field. If the result is smaller than the threshold, it is decided that the current field is of the same frame of the original film as the frame of the one-frame preceding field, that is, the duplicate field. The result of the comparison to the threshold is held in the hold circuit 600 for one-field period, and supplied to the control signal generator 700, which then generates the control signal corresponding to the telecine image. The frame memory 900 prevents the duplicate fields from being fed to the next stage. That is, the frame memory 900 converts the telecine image (30 Hz) into the frame rate of the film image (24 Hz) and feeds it to the output terminal 30 of the system. The write control circuit 800 controls the frame memory 900 to write, and the read control circuit 850 controls the frame memory 900 to read in accordance with the control signal fed from the control signal generator 700.
FIG. 12 shows the video signal conversion operation of the frame memory 900 illustrated in FIG. 11. The conversion operation for the telecine signal will be described with reference to FIG. 12. The telecine signal fed through the input terminal 10 as shown in FIG. 11 is passed through the memory 900 to the output terminal of the system in the order of successive fields (fields 1, 2, 3, 4 shown in FIG. 12A) until a duplicate field is detected. When a duplicate field is detected as shown in FIG. 12A at the latter one of the fields "3", this duplicate field is not transmitted, but the following fields (fields 6, 5, 8, 7 shown in FIG. 12A) are supplied to the output terminal. The control signal generator 700 generates a flag indicative of whether a duplicate field is detected or not (for example, RepeatFirstField in MPEG2 standard) and supplies it to the next stage.
Thus, the television signal conversion system constructed as above removes the duplicate fields from the telecine signal, and as a result produces at the output terminal 30 the video signal of film rate (24 Hz) with the duplicate fields removed as shown in FIG. 12B. Also, the control signal associated with the output video signal with the duplicate fields removed is produced from the output terminal 20 in synchronism with the output video signal as shown in FIG. 12C.
However, the conventional method for detecting the duplicate fields has the following problems.
(1) The noise included in the telecine signal has a large effect. This is because the result from the accumulator 400 is increased over a threshold by much noise included in the telecine signal, with a result that the field normally decided as a duplicate field may be regarded as being not a duplicate, or that the duplicate fields sometimes cannot be completely detected.
The problem (1) will be described in detail with reference to FIG. 13.
FIG. 13 is a graph of the output of the accumulator 400 in FIG. 11 as the ordinate with respect to the field count as the abscissa, with a certain telecine signal being supplied to the input terminal. Since the telecine signal produced as a result of conversion by 2-3 pull down system periodically includes duplicate fields as previously described, the output result from the accumulator 400 periodically decreases in accordance with the duplications.
As illustrated in FIG. 13, when a sequence (first scene) of the telecine signal including much noise and another sequence (second scene) of the telecine signal including less noise are continuously supplied, the following troubles are caused when the duplicate fields are tried to be detected by the conventional method. The duplicate fields in the first scene can be completely detected by comparing the result from the accumulator 400 to a first threshold, but all the fields in the second scene are regarded as duplicate fields since the result from the accumulator 400 is always smaller than the first threshold. In addition, if the result from the accumulator 400 is compared with a second threshold, the duplicate fields in the first scene cannot be detected at all, and all the fields in the second scene are decided to be duplicate fields. If the result from the accumulator 400 is compared with a third threshold, the duplicate fields in the first scene cannot be detected at all, but the duplicate fields in the second scene can be completely detected. In that case, the first to third thresholds are supplied to one input end of the comparator 500 in FIG. 11. Therefore, the duplicate fields in the telecine signal sometimes cannot be completely detected by the conventional method.
(2) There is a variable length coding method in which temporary encoding is first made for determining parameters for real encoding and then the real encoding is made. In this method, the duplicate fields detected and removed at the time of temporary encoding are required to coincide with those to be removed upon real encoding. If both are not coincident, the parameters for real encoding, determined at the time of temporary encoding are different from those in the real encoding.
On the other hand, if the telecine signal is reproduced from, for example, a recording medium on a VTR and supplied to the system, noise is sometimes caused in the telecine signal reproduced from the recording medium. At this time, even though the same telecine signal is tried to reproduce from the same recording medium in both temporary and real encoding operations, different telecine signals will be reproduced because of the inclusion of noise. Thus, the result of detecting the duplicate fields at the time of temporary encoding is sometimes different from that at the time of real encoding. Accordingly, the parameters in the following encoding means differ from those determined at the time of temporary encoding, with the result that the picture quality is considerably deteriorated upon decoding.
(3) Since each duplicate field is eliminated immediately after being detected, the fields of the television signal cannot be kept continuous each time a duplicate field is detected. Thus, when the fields which are close to the current field are tried to be processed on the basis of the frame unit according to MPEG2 standard or the like, the switching between even and odd fields will need the change of the processing order, making the processing complicated.
(4) The telecine signal is supplied at the frame rate of 30 Hz, while the converted signal has the frame rate of 24 Hz. Thus, different reference clocks must be separately provided for both the input telecine signal and the converted output signal or for the next stage, making the circuit operations unstable.
In addition, since this system always detects whether the current field is duplicate or not, once missing of, for example, this detection, due to noise or the like will result in successive detection of only even or odd duplicate fields. Thus, if the system employs the frame-unit processing of television signals as in the MPEG2 standard, the successive odd or even fields cannot be processed on the frame-unit basis, with the result that the encoding or transmission processing cannot be performed.
The above difficulties will be caused by the following factors.
(5) When the telecine images are directly edited by VTR or the like, the edited images have irregular conversion patterns unlike those shown in FIG. 12, or even or odd duplicate fields are sometimes successively detected. As a result, only even or odd fields are successively transmitted, and thus cannot be processed on the frame-unit basis according to MPEG2 standard.
(6) When the telecine images have successive frames of still images, the system always decides that the successive frames are a succession of duplicate fields. Thus, when the control flags (TopFieldFirst and RepeatFirstField) in the MPEG2 standard are used, the pattern cannot be controlled so that the encoding or transmission cannot be continuously performed.
(7) If the telecine signal having much noise is supplied to the system that uses a fixed threshold for determining whether the current field is duplicate or not, the difference value between frames having much noise is always larger than this threshold, so that the fields of those frames are always decided to be independent fields. Thus, since the duplicate fields cannot be completely detected and removed, the efficiency of the encoding or transmission can be increased.
(8) Moreover, when the telecine signal and the normal television signal (frame rate of 30 Hz) are mixed and have noise components changed, discrimination cannot be made between the actual change of the frame rate and the effect of noise on the decision of whether the current field is duplicate or not. Thus, when a duplicate field is decided to be an independent field by mistake, the amount of information to be transmitted to the next stage becomes redundant. If an independent field is decided to be a duplicate field by mistake, the picture quality is greatly deteriorated.